Re: [PATCH v2 2/2] mmc: core: sdio: skip CMD11 if signalling is already 1.8V

[Ulf Hansson]

On Tue, Oct 27, 2026 at 1:24 PM Deomid rojer Ryabkov <rojer@xxxxxxxx> wrote:
>
> Per the SDIO Simplified Specification Version 3.00, section 3.1.2, a card
> whose signalling is already at 1.8V must report S18A = 0 in the R4
> response, and the core relies on that: it only sends CMD11
> (SD_SWITCH_VOLTAGE) when S18A is set.
>
> Some cards don't follow this. The Realtek RTL8822CS (as fitted on the
> FriendlyElec NanoPi R2S Plus, where the M.2 SDIO bus is wired to a fixed
> 1.8V vqmmc) answers CMD5 with S18A = 1 even though the bus is already at
> 1.8V. CMD11 then times out:
>
>   dwmmc_rockchip ff5f0000.mmc: Unexpected CMD11 timeout
>   mmc_host mmc2: Timeout sending command (cmd 0x10202000 arg 0x0 status 0x90202000)
>   mmc2: new high speed SDIO card at address 0001
>
> and the card ends up in 50 MHz high-speed mode instead of SDR104.
>
> The SWT6621S misreports S18A the same way; there, the failed switch
> leaves the card unable to initialise at all (error -110) instead of
> falling back to high speed.
>
> When vqmmc can't do 3.3V, mmc_set_initial_signal_voltage() has already
> fallen back to 1.8V by the time CMD5 is sent, so the host knows there is
> nothing to switch. Skip CMD11 in that case. ocr keeps R4_18V_PRESENT, so
> mmc_sdio_init_uhs_card() still runs and the card is tuned to SDR104:
>
>   dwmmc_rockchip ff5f0000.mmc: Successfully tuned phase to 64
>   mmc2: new ultra high speed SDR104 SDIO card at address 0001
>
> Hosts that start at 3.3V are not affected, nor are compliant cards that
> report S18A = 0 at 1.8V. A non-compliant card that does need CMD11
> before entering UHS at 1.8V would now fail UHS init; with the previous
> patch it falls back to high-speed mode and keeps working.
>
> This relies on the fixed 1.8V supply being described as vqmmc-supply.
> vqmmc is optional: without it, the host can't tell that the bus is
> hardwired to 1.8V. Setting 3.3V then "succeeds", the core believes the
> bus is at 3.3V and CMD11 is still sent, so the patch doesn't help. Such
> boards need to describe the supply (e.g. a fixed 1.8V regulator as
> vqmmc-supply) to benefit from it.
>
> FriendlyElec's BSP for this board works around this with a card
> quirk, MMC_QUIRK_BROKEN_ROCR_S18A[1], matched on the "realtek,rtl8822cs"
> compatible, which drops the switch request whenever this chip is
> present. The misreported S18A is the card's fault, but ignoring it is
> only correct when the bus is already at 1.8V: on a board with a
> switchable vqmmc the same chip would then be run in UHS at 3.3V. A card
> quirk would need knowledge of the board's supply that it doesn't have.
> The core does know the current signal voltage, so handle it there, which
> also takes care of any other card that turns out to behave the same way.
>
> [1] https://github.com/friendlyarm/kernel-rockchip/commit/aee62cf7ae16b5ede6c6d92a8ee20e796482754e
> Cc: Jensen Huang <jensenhuang@xxxxxxxxxxxxxxx>
> Reviewed-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> Tested-by: Longjian Lin <llj@xxxxxxxxxxxxxx>
> Signed-off-by: Deomid rojer Ryabkov <rojer@xxxxxxxx>

This looks good to me, but deferring to apply until we sorted out the
potential issue in patch1.

Kind regards
Uffe

> ---
>  drivers/mmc/core/sdio.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/mmc/core/sdio.c b/drivers/mmc/core/sdio.c
> index 6787fdf1c53..a986ee04ba2 100644
> --- a/drivers/mmc/core/sdio.c
> +++ b/drivers/mmc/core/sdio.c
> @@ -737,9 +737,11 @@ static int mmc_sdio_init_card(struct mmc_host *host, u32 ocr,
>          * 1.8v, the card sets S18A to 0 in the R4 response. So it will
>          * fails to check rocr & R4_18V_PRESENT,  but we still need to
>          * try to init uhs card. sdio_read_cccr will take over this task
> -        * to make sure which speed mode should work.
> +        * to make sure which speed mode should work. Some cards (RTL8822CS)
> +        * set S18A regardless, so don't send CMD11 if we are already at 1.8V.
>          */
> -       if (rocr & ocr & R4_18V_PRESENT) {
> +       if ((rocr & ocr & R4_18V_PRESENT) &&
> +           host->ios.signal_voltage != MMC_SIGNAL_VOLTAGE_180) {
>                 err = mmc_set_uhs_voltage(host, ocr_card);
>                 if (err == -EAGAIN) {
>                         mmc_sdio_pre_init(host, ocr_card, card);
> --
> 2.53.0
>